Abstract:

Submarine canyon wall and tributary sequences at three Annot Sandstone localities in the French Maritime Alps record early-stage resedimentation events in proximal sectors of the Tertiary Annot Basin. Canyon margin lithofacies are distinctive in that they comprise a more variable suite of stratal types than intracanyon slope, canyon axis, distal fan and basin series of the same formation. Characteristic criteria include the highly variable geometry and spatial distribution of the series of strata, irregular bedding thickness, paleocurrent directions that diverge from the predominant regional patterns, and discontinuities within the formation and between the Annot Sandstone and the older marine shale series (Eocene Marnes bleues) forming the canyon margins. Three distinctive sandstone stratification types dominate the "gres d'Annot" canyon wall association: type 1 units, moderately to well-stratified and massive (often amalgamated), emplaced by debris flow and a continuum of sediment-fluid flow mechanism, not specifically identifiable in the field; some thick sand layers may represent deposition as "quick" beds from high-concentration underflows, possibly gradational between liquified and turbidity current flows; type 2 units, displaying slightly to extensive deformed horizons within but not throughout the beds, probably are related to liquefied flow and post-depositional liquefaction processes; and type 3 units, emplaced "en masse" and in some cases showing complete disruption of primary stratification (chaotic bedding), are identified as slides and slumps. In addition to the three above types, lower proportions of graded, generally thin "classic" sandstone turbidites (Ta-b, Tb-c, and Tb-e) and mudstone turbidites are recognized.
Although they appear as distinctly different entities in the field, a genetic relation between some depositional types is suggested. The mapped facies diversity is interpreted in terms of flow transformation, that is, the release of different sediment types along the dispersal path from a single sediment gravity flow as it evolves during its progression downslope. The diversity of mass flow products at the three canyon margin localities records a variable succession of transformation phases on the relatively steep slopes (locally in excess of (locally in excess of 10 degrees) within a short distance from the point of initial failure. The distinctive aspect of d'Annot" canyon margin sedimentation is the repetitive erosion->transport->deposition..pattern* of lateral infill. Definition of these proximal lithofacies serves to better understand the origin of the more distal marine fan and Annot Basin plain sequences seaward of the three canyon localities examined and also can be applicable to the study of modern canyon-fan settings.